1. Field of the Invention
The present invention relates to sensing devices that analyze physical properties of test subjects by means of electromagnetic waves in order to obtain information on the test subjects. In particular, the present invention relates to a sensing device that employs an electromagnetic wave (also referred to as a terahertz wave) within a frequency range of a millimeter-wave band to a terahertz band (30 GHz to 30 THz) in order to obtain information on a test subject.
2. Description of the Related Art
As devices for generating and detecting terahertz waves have been developed in recent years, technologies employing terahertz waves have been drawing much attention. For example, as an application field of terahertz waves, there have been researched and developed a technology for performing imaging using safe fluoroscopic devices to replace devices that employ X-rays, a spectroscopic technology for examining a complex dielectric constant or absorption spectrum of a material to check the bonding condition, a technology for analyzing biomolecules, and a communication technology.
Japanese Patent Laid-Open No. 8-320254 (corresponding to U.S. Pat. Nos. 5,710,430, 5,623,145) discloses a spectroscopic analysis device equipped with a spatial optical system that employs terahertz waves. Specifically, in this device, a terahertz wave is emitted towards a test subject, and a transmitted wave signal is measured by time-domain spectroscopy (THz-TDS). The signal is compared with a signal obtained when the test subject was not present (i.e. a reference signal) so as to determine the properties of the test subject from propagation delay and absorption spectrum. By scanning the terahertz wave or the test subject, a two-dimensional imaging operation can be performed.
Japanese Patent Laid-Open No. 2000-89042 discloses an optical fiber sensor that detects a leakage of a liquid. Specifically, an optical fiber is given a curved portion so that an electromagnetic wave exceeding a critical angle at the curved portion is forced to leak from a core layer. This electromagnetic wave and a test subject penetrated in a resin coating layer disposed on the exterior of a cladding layer are allowed to interact with each other, whereby the test subject can be detected. Furthermore, Nature, Vol. 432, p. 376, 2004 discloses a technology for allowing a terahertz wave to propagate through a single line, and discusses the low-loss and low-dispersion properties of a single line within the terahertz-wave band.
However, with regard to Japanese Patent Laid-Open No. 8-320254, since the device uses a spatial optical system for the analysis of the test subject, the analysis may be easily affected by moisture in the air. This is because an energy band that corresponds to modes such as vibration and rotation of water molecules in the moisture vapor is present within the terahertz-wave band, causing the terahertz wave to be absorbed by the water molecules. Therefore, in a spatial optical system that employs terahertz waves, the propagation of a terahertz wave is usually performed within a nitrogen atmosphere or in vacuum. However, since the terahertz waves have properties in which they are easily susceptible to moisture in the air, an analysis technique that can reduce the effect of moisture in the air and increase the electromagnetic-wave propagation density to allow the electromagnetic waves to extend over a wider band is in demand.
On the other hand, Japanese Patent Laid-Open No. 2000-89042 is merely a disclosure of a technology that employs the transmissibility of light through an optical fiber constituted by a core layer and a cladding layer. Likewise, Nature, Vol. 432, p. 376, 2004 is merely a disclosure of a wire transmission path formed of a single conductor.